Co-channel interference (CCI) is constituted by one or more interfering signals within the frequency band of a desired signal in a communications system, and is a key factor that limits the frequency reuse capacity of mobile communications systems. The CCI can not be reduced by conventional filtering techniques, because it falls within the bandwidth of the desired signal. Until now, it has been necessary in communications systems to provide a relatively high ratio of the desired signal strength to the interfering signal strength, generally referred to as the signal-to-interference or C/I ratio, in order to maintain effective communications. Little has been done to reduce CCI from a desired signal.
Harrington U.S. Pat. No. 5,084,899 issued Jan. 28, 1992 and entitled "Signal Suppressors" discloses an interference suppressor in which an amplitude limiter and bandpass filter is used to enhance the amplitude difference between a wanted signal and a dominant (i.e. greater amplitude) interfering signal, to enable the interfering signal to be subtracted from a received signal to leave only the wanted signal. This is an analog signal arrangement, and only works when the interfering signal is larger than the wanted signal, preferably by a large margin such as 18 dB, such as may be the case for example in a spread spectrum communications system.
An object of this invention is to provide methods and apparatus for reducing CCI from a desired signal.